1. Field of the Invention
The present invention relates to a cup-type harmonic drive having a rigid circular member, a flexible cup member and a wave generator, and more particularly to a cup-type harmonic drive having a shorter flexible cup member.
2. Description of Prior Art
A cup-type harmonic gear drive has been known as one of typical harmonic drives, which typically consists of a rigid circular spline, a cup-shaped flexspline having 2n (n being a positive integer) less teeth than the circular spline and being disposed inside the circular spline and flexed into an elliptical shape so as to engage with the circular spline at two places, and a wave generator fitted inside the flexspline for flexing it.
Rotating the input shaft of the wave generator rotates the elliptical shape of the flexspline and causes the flexspline and circular spline to rotate relative to each other in proportion to the difference in the number of their teeth. If one of the circular spline and flexspline is fixed and the other is connected with an output shaft, the output shaft will rotate at a greatly reduced speed relative to the input shaft. Because of this ability to produce a large reduction ratio with only a small number of machine elements, harmonic gear drives are widely applied in precision machines and the like.
The harmonic gear drives are disclosed, for example, in U.S. Pat. No. 2,906,143 issued to Musser on Sep. 29, 1959, U.S. Pat. No. 4,823,638 issued to Ishikawa on Apr. 25, 1989 and U.S. Pat. No. 4,974,470 issued also to Ishikawa on Dec. 4, 1990 and the like.
As illustrated in FIG. 1 wherein an embodiment of the present invention is shown, the cup-shaped flexspline 3 of the cup-type harmonic gear drive 1 consists of a tube portion 31, a disc-shaped diaphragm portion 32 connected integrally on one end of the tube portion 31 to define the cup bottom, and a thick boss portion 33 formed integrally on the center portion of the diaphragm 32. The flexspline teeth 34 are formed on the outer surface adjacent to the open end of the tube portion 31. In operation, the flexspline 3 is repeatedly flexed in the radial direction at the side of its open end by means of the wave generator 4. The tube and diaphragm portions 31, 32 serve as a kind of flexible coupling to absorb deformation of the teeth portion of the tube which is deformed into an ellipoid by the wave generator 4.
The amount of elliptical deformation of the tooth portion is generally determined in view of mechanism on the basis of the reduction ratio. Thus, in the conventional harmonic gear drives, the ratio of the pitch diameter of the tooth portion to the axial length of the flexspline is approximately 1:1. Reducing the axial length of the flexspline causes increase of the coning angle of the flexspline 3 as shown in FIG. 10. The increase in the coning angle causes the bending stress repeatedly appeared in the diaphragm 32 to increase, and also causes to reduce the engagement region between the teeth of the splines. These deteriorate the strength of the harmonic gear drive. In addition, as illustrated in FIG. 19, the misalignment of the flexspline setting will greatly deteriorate the performance of the harmonic gear drive.
Further, increase of the coning angle of the flexspline causes to shift the engagement region of the teeth between the flexspline and the circular spline. As illustrated in FIG. 11, where the coning angle .THETA. is sufficiently small, the teeth of the flexspline engage with those of the circular spline in the region A designated by diagonal lines. The wave bearing 43 is positioned substantially in accordance with this engagement region A. In contrast, where the coning angle is larger, the teeth of the flexspline engage only at their end side portion with those of the circular spline in the narrow extent of the engagement region A as shown in FIG. 12. Thus, the engagement region A is shifted from the position where the wave bearing 43 is disposed. In this condition, since the engagement of the teeth between the flexspline and the circular spline is no longer supported or maintained properly by the wave bearing, proper torque transfer through the engagement region cannot be expected. In the prior art, this kind of problem has not been fully recognized, and so no proposal has been made to solve the problem.
Accordingly, one object of the present invention is to provide a harmonic drive which has a cup-shaped flexible member of an improved configuration so that a compact harmonic drive having a shorter flexspline can easily be realized.
Another object of the present invention is to provide a harmonic drive in which a circular rigid member and a flexible member thereof engage properly with each other even where a shorter flexible member is adopted.